The present invention relates to an optical switch which might be used, for example, as a demultiplexer for adding or dropping OTDM (optical time-division multiplexing)channels in an optical telecommunications system.
It has been recognised that in order to achieve the highest possible bit rates (e.g. of 100 Cbit/s or more) in optical transmission systems it is desirable that functions such as adding or dropping a channel should be carried out entirely in the optical domain. It has previously been proposed to carry out such functions using a non-linear loop mirror (NOLM) in which the non-linearity is provided by a semiconductor gain medium which is positioned asymmetrically in the loop. Incoming optical data is split into two counter-propagating components. Because the semiconductor gain medium is positioned asymmetrically, one of these components passes through it before the other. Accordingly, if an optical control signal is applied to the gain medium to switch it after the first component has passed through but before the second counter-propagating component has arrived, then the two components experience different phase shifts through the semiconductor gain medium, producing a differential phase shift which may used to switch the signal to determine whether it is transmitted or reflected by the NOLM.
Although, as described in the present applicant's co-pending International application PCT/GB94/02240, it is found to be possible to operate such as NOLM at switching rates higher than the saturation recovery time of the semiconductor optical gain medium, nonetheless there remains a fundamental upper limit to the possible switching speeds determined largely by the transit time associated with the semiconductor gain medium. In use, there has to be time for the first pulse to pass through the gain medium, then for the gating pulse to pass through, and subsequently for the second pulse to pass through the medium. This limits the possible bit-rate which can be handled by the switch.